The invention is directed to a connecting device for plastic tubes, particularly for multi-layered compound tubes and tubes of cross-linked or not cross-linked plastic material with or without an oxygen diffusion barrier layer according to the preamble of claim 1 and a method for connecting such a plastic tube to a connecting body.
Previously, for connecting or joining plastic tubes to or at connecting bodies, there have been used connecting devices known from the technology for connecting and joining metal tubes. In these known plastic tube connecting devices, the connecting body, being e.g. a connector for connecting two or more plastic tubes, a T-shaped or angular member or a distributor means (so-called fittings) or another fixedly mounted member adapted for connection of the plastic tube, comprises a connecting piece adapted for plugging attachment of the end portion of the plastic tube to be connected. To this purpose, it may be required to widen the end portion of the plastic tube prior to plugging attachment.
However, there are also known connecting devices wherein the outer diameter of the connecting piece is smaller than the inner diameter of the plastic tube. The peripheral surface of the connecting pieces is profiled in a specific manner, particularly by being provided with saw-tooth-shaped peripheral ribs. By means of a union nut having an inner cone to be screwed with the connecting body, the end portion of the plastic tube plugged onto the connecting piece is pressed against the connecting piece, with the peripheral ribs becoming "hooked" to the plastic tube. Other connecting devices are provided with a slit ring having an inner profile which is upset during tightening of the union nut and in this manner presses the plastic tube against the peripheral surface of the connecting piece. A disadvantage of known connecting devices for plastic tubes resides in that the profiles of the connecting piece and the slit ring can damage the plastic tube. This can lead either to leakages in the plastic tube or, in multi-layered compound tubes, to damage of the plastic coating, the result being that the core of the metal tube is not protected against corrosion anymore. Tightening of the union nut causes an increase of the contact pressure by which the plastic tube is pressed against the connecting piece or the slit ring is pressed against the plastic tube. Starting with a specific contact pressure, the plastic tube will be rotated together with the union nut, leading to twisting of the plastic tube on the one hand and causing the profile to "cut" into the plastic tube on the other hand. Further, after all, it is merely the profile in the region of the free end of the connecting piece that provides for the attachment of the plastic tube to the connecting body during tensile stresses. This protection against detachment of the plastic tube from the connecting device might not be sufficient in case of relatively high tensile stresses. Also, due to the profiles of the connecting piece or the slit ring, shearing forces will occur in this kind of "anchoring" of the plastic tube when the plastic tube is subjected to tensile forces.
In a two-part clamping connector known from DE 38 36 124 A1, a pressing sleeve is provided to be pressed onto the connecting end of a tube previously slipped on a connecting piece while being widened. The tube is secured to the connecting piece by continuous peripheral projections of the connecting piece between which the material of the plastic tube is pressed when the pressing sleeve is pushed onto the tube. That peripheral projection which is most remote from the free end of the connecting piece is higher than the other projections so that the securing of the plastic tube is accomplished particularly in this region. Since the plugged plastic tube is moved merely to a comparatively small extent over the highest projection, there is a danger of damages to the plastic tube due to shearing forces when the tube is subjected to axial tension.